Hydrocarbon‐bridged Metal Complexes, XXI. – Addition of Tetracarbonylosmate and ‐ruthenate to Coordinated Unsaturated Hydrocarbons: Trinuclear Complexes with two σ,α‐Allyl, Methylallyl, Cyclohexadiene, Cycloheptadiene, Cyclohexadienyl, Cycloheptatrien, and “Thiophene” Bridges The addition of organometallic nucleophiles [M(CO) 4 ] 2‐ (M = Os, Ru) to [(Cp)(ON)(OC)Mo(allyl)] + , [Cp(OC) 2 Mo(butadiene)] + , [(OC) 3 Fe(cycloheptadienyl)] + , [(OC) 3 Fe(cycloheptadienyl)] + , [(OC) 3 Mn(benzene)] + , [(OC) 3 Mn(thiophene)] + , [(OC) 3 ‐Mo(cycloheptatrienyl)] + leads to the heterotrimetallic η 1 :η 2 ‐, η 1 :η 3 ‐, η 1 :η 4 ‐, η 1 :η 5 ‐, η 1 :η 6 ‐hydrocarbon‐bridged complexes 1 ‐ 7. The structures of cis ‐(OC) 4 Os[(μ‐η 1 :η 4 ‐C 7 H 9 )Fe(CO) 3 ] 2 (4), cis ‐(OC) 4 Os[(μ‐η 1 : 5 ‐C 6 H 6 ‐Mn(CO) 3 ] 2 (6), and cis ‐(OC) 4 Os‐[μ‐η 1 :η 6 ‐C 7 H 7 ‐Mo(CO) 3 ] 2 (7b) have been determined by X‐ray analysis. The fluxional behaviour of 6 in solution has been studied in a temperature range from – 70 to +60°C. The comparison of observed and calculated 1 H‐NMR data indicates a 1,2‐shift mechanism (ΔG + = 49 kJ/mol).